1. Field of Invention
The present invention relates to a pixel reset level control, and more particularly, to reducing image lag in CMOS active pixel sensors at low light levels by controlling the reset level.
2. Description of Related Art
Image acquisition can be achieved by various semiconductors. Some of the conventional devices are charge-coupled devices (CCDs), photodiode arrays, charge injection devices and hybrid focal plane arrays. Of these CCDs are frequently implemented, due to the fact that they represent a mature technology, are capable of large formats and very small pixel size and they facilitate noise-reduced charge domain processing techniques such as binning and time delay integration.
However, CCD imagers suffer from a number of drawbacks. For example, the signal fidelity of a CCD decreases as the charge transfer efficiency is raised to the power of the number of stages. Since CCDs use many stages, the CCD fabrication technique needs to be optimized for very efficient charge transfer efficiency. CCDs are also susceptible to radiation damage, require good light shielding to avoid smear and have high power dissipation for large arrays.
The image signal processing electronics required for the imager can be fabricated in CMOS. Each column of CCD pixels is transferred to a corresponding cell of a serial output register before being processed in off-chip signal processing electronics. This architecture limits the read-out frame rate, which an amplifier can handle proportional to the number of charge packets divided by the number of pixels in the imager.
Other types of imager devices have problems as well. Photodiode arrays can exhibit high kT/C noise. Photodiode arrays also suffer from lag. Charge injection devices also can have high noise. Hybrid focal plane arrays exhibit less noise but are prohibitively expensive for many applications and have relatively small array sizes.
Therefore, there is need for an improved image sensor that provides a reduced image lag in CMOS active pixel sensors at low light levels.